Refining aluminum electrolytically



Patented Jan. 5, 1954 REFINING ALUMINUM ELECTROLYTICALLY Charles Daniel Mngoz, Saint-Jean-de-Maurienne, France, assignor to Pechiney Compagnie de Produits Chimiques et Electrometallurgiques, a corporation of France No Drawing. Application June 12, 1950, Serial No. 167,695

This invention relates to improvements in refining aluminum andmore particularly to an electrolyte to be used in a refining process Wherein the electrolyte floats, in a molten state, upon a molten alloy with other substances of the aluminum to be refined, the alloy being used as an anode and the purified aluminum removed therefrom being deposited on a layer of molten alu minum which floats on the electrolyte and is used as a cathode.

The electrolyte of the present invention provides for the minimizing of undesirable deposits on the electrodes of an aluminum refining apparatus.

The instant invention further provides a bath for the electrolytic refining of aluminum that remains constant in composition during long periods, whereby replenishing the solution is sim plified. v

In addition, the bath of the instant invention is such as to minimize the deterioration of the walls of the tank in which the process is being carried on.

The bath of the instant invention provides for very little occluded or dissolved gas in the refined aluminum.

Furthermore, the bath of the instant invention provides for considerably less deposit of crust on the side walls of the tank than is found in the use of previously-known electrolytes.

Furthermore, the use of the bath of the instant invention enables the use of a tank having superior thermal insulating properties, with a resultant lowering of heat loss and better electrical and thermal efiiciency.

With the foregoing and other objects in view which will appear as the description proceeds, the invention resides in the methods and compositions and in the details thereof hereinafter described and claimed, it being understood that Changes in the precise embodiment of the invention herein disclosed may be made without departing from the spirit of the invention.

Refined aluminum having a content of 99.99% of pure aluminum is obtained by means of electrolysis in a cell comprising three superposed layers of molten material. The lower layer is used as an anode and is generally an alloy of the unrefined, commercial aluminum With a heavy metal such as copper. A layer of molten material floats upon the molten alloy and serves as an electrolyte. A layer of molten refined aluminum floats on the electrolyte and is used as a cathode, graphite electrodes being inserted into this layer for that purpose. Any material used as an electrolyte 3 Claims. (01. 204-67) in such a process must possess two qualities: it must have a definite melting point and it must be selective in order to obtain very pure refined aluminum. In the past, the electrolytes used contained fluorides'and particularly sodium fluoride. For instance, one type of electrolyte generally used is composed of a mixture of barium chloridewith aluminum fluoride and sodium fluoride; theproportion of A1F3 to Na]? being larger than in.

natural cryolite; a small amount of sodium chloride is added to promote stability. Such a bath.

has a composition somewhat as follows:

However, a bath such as described above, and particularly one containing halides of alkali metals such as sodium or potassium, has many disadvantages. For instance, the lower part of the graphite electrodes which are immersed in the cathode of molten refined aluminum are subject to being attacked by sodium and the like. An insulating deposit is thus formed which prevents flow of electrical current through the solution. If this deposit is allowed to remain during the operation of the process, there will be many obvious electrical difficulties, such as dangerous. overcharging of the anode.

A further diificulty encountered in such prior art electrolytic baths is that the aluminum fluoride is evaporated more than the other components. The composition of the bath must be regu-- larly corrected by the addition of a mixture of salts rich in aluminum fluoride. As a practical. matter, about 15 kgs. of BaClz, 15 kgs. of AlFz, and 5 kgs. of NaCl have to be added every three days to a cell using 14,000 amperes. This mixture melts at more than 800 C., is very unstable, and cannot be easily purified by electrolysis in a feeding pot before being introduced into the refining cell proper. However, the aluminum fluoride is so impure as to require refining, and it becomes necessary to employ a complicated and generally undesirable method, the details of which are immaterial to the instant application, but which is to be avoided if possible.

Another difficulty encountered in the electrolytes containing halides of, alkali metals is their effect on the tank in which the process is carried on. The side walls of these tanks are generally made of magnesium bricks and are rapidly attacked in the portions where they are cell operates between 750 C. and 800 C. HOW-a ever, there are parts of the bath that remain liquid down to 630 C. and thefieliquid parts seep. into cracks in the walls of the. tank and, when they reach isothermal surfacescorresponding to this temperature, the resultis deep errosiono the masonry. This is quite important in efi ect upon the thermal insulation properties of the wall and, thus, upon the thermal eificiency oi-theprocess.

In addition, the prior art baths described-above result in refined aluminum that contains undesirable occluded or dissolved gases.

By use of the electrolytic bath oi the applicant's invention, it is possible to refine aluminum, and,

at the same time, do away with the drawbacks inherent in those electrolytescontainingaluminum fluoride and compounds, particularly halides, of

alkali metals. While baths generallyused in the refining of aluminumalways contain these objectionable salts and their presence was always thought to be necessary, the applicant discovered the surprising fact that far better results in every way are obtained with particular compositions of electrolyte without sodium and other alkali metals salts present. The baths invented by the applicant enable the'cells to operate atthe usual temperatures between 700 C. and-800 C. with electrical efiiciencies as high asandwith the .produotionof refined aluminum as pure as with the useof known baths, without the difnculties inherent in the latter. According to the present invention, electrolytic baths of the following genoral-composition are used:

Percent .by. weight 'A bathof thiscomposition is-more-stable than one containing sodium 'halides -and the like. In fact, the appearance of the applicant's bath is entirely different from former baths: itis more transparent, which signifies that itcontains less oxide formed by decomposition of thesalts which comprise the mixture. For this reason,'less crust is formed on the side walls of the tank, particularly at the level of the electrolytic-layer. examination of the compositions possible within the scope of the applicant's invention will show that each of the components, namely BaClz,CaFz, and AlFz, may vary within limits of of-the-weight of the mixture; the result is that the-ratioof aluminum fluoride to calcium fiuoride'may vary from 09 to 2.0.

An example-of a bath-Within-the scopeoi this invention is as'follows:

Percent byweight 42 CaFz 28 AlEs BaFz, MgFz, oxides, and otherimpuritiesufl 5 A bath of this composition yieldsan aluminum whichis'99.99,.% pure and' has a ratio of aluminum fluoride tocalciumfiuoride of 0.9.

Another example. of a *bath within the;scope or this invention and which is richer in aluminum fluoride is as follows:

Percent by weight BaClz 42 CaFz 18 AIR; BaFz, MgFz, oxides, and other impurities 5 A bath of this composition will operate for several weeks, but loses aluminum fiuoride until the bath reaches apointof stable composition wherein the aluminum fluoride content is 30 Whena bath which acts with high selectivity with respect toaluminum is desired, it is preferable-touse an aluminum fluoride content which i isnot-appreciably. lower than the content of calciurn fluoride If too small an amount of alumifluoride is used, deposits of calcium or barium may develop on the cathode. It has been found-that very good results are obtained when the percentages of aluminum and calcium fluoridesare chosen so that the ratio of AlEa to CaFz is .in theneighborhoodoi 1.5, say 1.3 to 1.6. Sucha bath whichoperates very satisfactorily is as follows:

Percentby weight BaClz CaFz 20 AlFs 30 BaFz, MgF2, oxides, and other impurities .5

Itcan be seen that the ratio of AlFz-to CaFz here is 1.5; this bath is very stable and does not change in composition during the electrolysis.

Baths of the above compositions within the scope of the invention and, in general, chloride and fluoride baths, or merely fluoride baths, which arefree of alkali metals, and, in particular, substantially free of sodium have many advantages which in part-will be outlined below.

In the use of the baths of the invention there isno considerable deposit on the faces of the graphite electrodes immersed in the liquid aluminum cathode. The number of times that these electrodes mustbe scraped is diminished by twothirds. The ,electricalcontact between thefaluminum and the graphite is considerably better. and again of 0.2 volt may be obtainedwhen the current density in the'graphite is, for example, in the order of 3;5-amperes per sq. cm.

In addition, a bathwithin the philosophy of the instant invention is very stable, and the total consumption of chloride-fluoride compounds is only about per centof the consumption'where the prior-arttype of bath is used; that is to sayabout 7kg. daily instead of 12kg, when operating with a 1,,0 00 ampere cell. Moreover, the composition of the bath remains constant over a given length of time, which;enables one to feed the cell with material always having the same composition. This material to'be added ,can be melted andrefined electrolytically in a feeding-cell before being added .to'the rnain refining cell, principally because of theabsenceof sodium compounds and dislike.

The use of the applicants bath cutsdown the deterioration of wall-sci the tank, this being due, as'has been explained, to the absence of sodium and the like. Baths following the teaching of the instant invention have substantially narrower solidification ranges than known baths containing alkali metal salts; for example, the above-described bath containing 45% -BaC12, 2o %;oaF2, 30% AlF and'5% of 'BaFg, grt', and oxides 'is-ch aracterized by a solidification range from l10 -'C.-to 680 C. as :coinparedwlth a range of from 700 C. to 630 0. exhibited by the usual prior-art type of bath. At the same time, the applicants baths have higher surface tension than in known electrolytes; for instance, the mixture whose composition is recited imme diately above has a surface tension of 148 dynes/cm. at 780 C. as compared with 118 dynes/cm. for the described prior-art bath containing halides of alkali metals. The two properties recited above as residing in baths of the composition taught by the present invention, that is, narrower range of solidification and higher surface tension, have an important effeet on the action of the baths on the lining, bricks and masonry, of the tank. The penetration of the bath into the cracks in the masonry is not as great as in the case of the previously-known baths; tanks having strong thermal insulation properties may be constructed without the magnesia linings being subjected to rapid deterioration; such tanks, thus, can be built to produce an electrolytic cell which requires less electric current density, less voltage at the terminals and less power consumption because of the lower heat loss through the walls of the tank.

Furthermore, refined aluminum obtained in a bath free of sodium and the like does not contain as much occluded or dissolved gases as aluminum obtained by the previously-known means. Moreover, the cells using baths according to the instant invention, may be fed melted-down chloride-fluoride or straight fluoride compounds. This means that the danger inherent in powdered materials of absorbing water from the air is avoided; the presence of water in powders which are to be introduced directly into a molten-metal bath are obvious. Another advantage resulting from the above-recited properties of the baths of the applicant is the minimizing of crusts of frozen bath that generally form upon the top layer in refining operations, and which must be frequently removed, taking a considerable portion of bath material along with it.

It is evident that in the baths of the instant invention, the composition is critical only with respect to the barium chloride, the calcium fluoride, and the aluminum fluoride; in using commercial materials of this kind, various impurities are certain to be present. We have recited above that barium fluoride, mangnesium fluoride, and various oxides of metals will be present; it is true, of course, that commercial barium chloride will contain some strontium salt and that almost every salt will contain small amounts of the almost universal sodium and potassium compounds. Consequently, it is contemplated that the baths of the applicant must necessarily contain small amounts of these compounds. However, it is obvious that a bath of the type taught by the applicant having as much as 1% of sodium chloride or sodium fluoride as accidental impurities is within the scope of the applicant's invention and is completely different from previously-known baths containing very much higher amounts of these undesirable compounds; that is, at least 5% and usually greater than 15%.

It will be evident to a worker in the art that, although certain salts may be selected within the scope of this invention that will operate properly chemically and electrically, care must be taken to select only compounds of such kind and amount as will produce an electrolytic bath whose specific gravity is such that it will float on the anodic layer and not float on the cathodic layer. The bath mixtures given as examples have this necessary quality and it is contemplated that anyone skilled in the art will be able to select compounds within the scope of the invention in the proper proportions to maintain such a desired quality.

It is to be understood that the invention is not limited to details herein specifically set forth but can be carried out in other ways without departure from its spirit as defined by the appended claims.

I claim:

1. A method of refining aluminum electrolytically by floating molten refined aluminum for use as a cathode on an electrolytic bath which in turn floats on a molten alloy of unrefined aluminum and a heavy metal for use as an anode, said bath comprising approximately 30 per cent by weight of aluminum fluoride, approximately 20 per cent by weight of calcium fluoride, 45 per cent by weight of barium chloride, the remainder being made up of barium fluoride, magnesium fluoride, oxides, and other impurities, there being no more than 1 per cent of sodium and potassium compounds, said fluoride and chlorides being selected in such proportion that the bath will float on the anodic layer but not float on the cathodic layer of a refining apparatus of the type described.

2. A method for the refining of aluminum electrolytically by floating molten refined aluminum for use as a cathode on an electrolytic bath which in turn floats on a molten alloy of unrefined aluminum and a heavy metal for use as an anode, said bath comprising from 37 to 47 per cent by weight of barium chloride, from 18 to 28 per cent by weight of calcium fluoride, the total amount of the three salts making up approximately per cent by weight of the bath, from 25 to 35 per cent by weight of aluminum fluoride, and containing no more than 1 per cent of alkali metal compounds, said fluorides and chlorides being selected in such proportion that the bath will float on the anodic layer but not float on the cathodic layer of a refining apparatus of the type described.

3. A method of refining aluminum electrolytically by floating molten refined aluminum for use as a cathode on an electrolytic bath which in turn floats on a molten alloy of unrefined aluminum and a heavy metal for use as an anode, said bath comprising from 37 to 4'7 per cent by weight of barium chloride, from 18 to 28 per cent by weight of calcium fluoride and from 25 to 35 per cent by weight of aluminum fluoride, the total amount of the three salts making up approximately 95 per cent by weight of the bath, there being no more than 1 per cent of sodium potassium compounds, said fluorides and chlorides being selected in such proportion that the bath will float on the anodic layer but not float on the cathodic layer of a refining apparatus of the type described.

CHARLES DANIEL Mrimidoz.

References Cited in the file of this patent UNITED STATES PATENTS Number 

2. A METHOD FOR THE REFINING OF ALUMINUM ELECTROLYTICALLY BY FLOATING MOLTEN REFINED ALUMINUM FOR USE AS A CATHODE ON AN ELECTROLYTIC BATH WHICH IN TURN FLOATS ON A MOLTEN ALLOY OF UNREFINED ALUMINUM AND A HEAVY METAL FOR USE AS AN ANODE, SAID BATH COMPRISING FROM 37 TO 47 PER CENT BY WEIGHT OF BARIUM CHLORIDE, FROM 18 TO 28 PERCENT BY WEIGHT OF CALCIUM FLUORIDE, THE TOTAL AMOUNT OF THE THREE SALTS MAKING UP APPROXIMATELY 95 PER CENT BY WEIGHT OF THE BATH, FROM 25 TO 35 PER CENT BY WEIGHT OF ALUMINUM FLUORIDE, AND CONTAINING NO MORE THAN 1 PER CENT OF ALKALI METAL COMPOUNDS, SAID FLUORIDES AND CHLORIDES BEING SELECTED IN SUCH PROPORTION THAT THE BATH WILL FLOAT ON THE ANODIC LAYER BUT NOT FLOAT ON THE CATHODIC LAYER OF A REFINING APPARATUS OF THE TYPE DESCRIBED. 